Hypoxia-induced pRB hypophosphorylation results from downregulation of CDK and upregulation of PP1 activities

Abstract

Exposure of CV-1P cells to hypoxic conditions results in reversible cell cycle arrest concomitant with accumulation of pRB in the hypophosphorylated, growth suppressive form. Similar to cell cycle arrest induced by serum starvation, we show here that hypoxia-induced arrest is accompanied by a decrease in pRB-directed CDK4 and CDK2 activities, lower cyclin D and E protein levels, and by an increase in p27 protein abundance. Immunoprecipitation studies reveal an increase in p27 association with cyclin E-CDK2 complexes. In contrast to cell cycle arrest induced by serum starvation, hypoxia increases PP1-mediated pRB dephosphorylation. These data reveal that synergy between decreased pRB-directed cyclin/CDK activity and increased pRB-directed phosphatase activity contribute towards inducing and maintaining pRB in its hypophosphorylated, growth suppressive state during hypoxia.

Oxygen deprivation causes suspended animation in the zebrafish embryo

“…This result contrasts with mammalian cell work that shows the majority of cells arrest in G 0 ͞G 1 (28)(29)(30)(31). There are at least two possible explanations for the S and G 2 phase arrest we see in zebrafish.…”

mdm2: a bridge over the two tumour suppressors, p53 and Rb

“…Although the ability of Rb to regulate the apoptotic function of p53 may not play a vital role during the normal development, it could be important in response to cellular stress. In line with this, it has been shown recently that stress signals such as hypoxia can induce hypophosphorylation of Rb (Amellem et al, 1996;Krtolica et al, 1998). Interestingly, mdm2 preferentially binds to the hypophosphorylated form of Rb (Hsieh et al, 1999;Xiao et al, 1995).…”

Nature of the Critical Labile Event That Controls RB Phosphorylation in the Cyclic AMP-Dependent Cell Cycle of Thyrocytes in Primary Culture

“…However, using the PY20 antibody we failed to detect the presence of phosphotyrosine in CDK4 immunoprecipitates of quiescent dog thyrocytes (negative data not shown). Alternatively, a modulation of RB binding to cyclin D has not been considered and the stimulation of type 1 phosphatase dephosphorylates RB at mitosis but is not instrumental in cell cycle arrest induced by serum starvation [39]. Finally, an independent regulation through cAMP of downstream G1 events, such as cyclin E-CDK2 activity [11], is not formally excluded by the present experiments.…”